U.S. patent number 7,843,262 [Application Number 12/351,876] was granted by the patent office on 2010-11-30 for high efficiency power amplifier.
This patent grant is currently assigned to NEC Electronics Corporation. Invention is credited to Isao Takenaka.
United States Patent |
7,843,262 |
Takenaka |
November 30, 2010 |
High efficiency power amplifier
Abstract
Disclosed a power amplifier including a main amplifier with
class bias AB and a peak amplifier with class C bias. A
quarter-wave length transmission line having a length equal to
one-fourth of the wave-length of a fundamental frequency is
connected to an output side of the peak amplifier. Outputs of the
main amplifier and the peak amplifier are combined. An envelope
amplifier that modulates the drain bias voltage in accordance with
an envelope of the modulation wave input signal and an envelope
detector are provided as a drain-bias circuit for the main
amplifier (FIG. 1).
Inventors: |
Takenaka; Isao (Kanagawa,
JP) |
Assignee: |
NEC Electronics Corporation
(Kanagawa, JP)
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Family
ID: |
40850120 |
Appl.
No.: |
12/351,876 |
Filed: |
January 12, 2009 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090179703 A1 |
Jul 16, 2009 |
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Foreign Application Priority Data
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Jan 15, 2008 [JP] |
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2008-006086 |
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Current U.S.
Class: |
330/136;
330/124R |
Current CPC
Class: |
H03F
1/0288 (20130101); H03F 3/602 (20130101); H03F
1/0222 (20130101); H03F 3/191 (20130101); H03F
2200/451 (20130101); H03F 2200/99 (20130101); H03F
2200/102 (20130101); H03F 2200/15 (20130101); H03F
2200/192 (20130101); H03F 2200/423 (20130101) |
Current International
Class: |
H03G
3/20 (20060101) |
Field of
Search: |
;330/136,124R,295 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Donald F. Kimball et al., "High-Efficiency Envelope-Tracking W-CDMA
Base Station Amplifier Using GaN HFETs", IEEE Transactions on
Microwave Theory and Techniques, vol. 54, No. 11, Nov. 2006, pp.
3848-3856. cited by other .
I. Takenaka et al., "A 240W Doherty GaAs Power FET Amplifier With
High Efficiency and Low Distortion for W-CDMA Base Stations", 2004
IEEE MTT-S Int. Microwave Symp. Dig., pp. 525-528. cited by
other.
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Primary Examiner: Choe; Henry K
Attorney, Agent or Firm: Young & Thompson
Claims
What is claimed is:
1. A power amplifier comprising: a main amplifier; a peak amplifier
having a fixed bias; and a quarter-wave length transmission line
connected to an output of the peak amplifier, the quarter-wave
length transmission line having a length equal to one-fourth of the
wave-length of a fundamental frequency; the main amplifier
including a drain bias circuit, the drain-bias circuit including:
an envelope detector that receives a modulation wave input signal
to detect an envelope thereof; and an envelope amplifier that
modulates a drain bias of the main amplifier in accordance with the
envelope of the modulation wave input signal.
2. A power amplifier comprising: a main amplifier; a peak amplifier
having a fixed bias; an input distributor that distributes the
power of a modulation wave input signal to the main amplifier and
to the peak amplifier; a quarter-wave length transmission line
connected to an output of the peak amplifier, the quarter-wave
length transmission line having a length equal to one-fourth of the
wave-length of a fundamental frequency; and a bias circuit that
supplies a bias voltage to the main amplifier, the bias circuit
including: an envelope detection circuit that receives the
modulation wave input signal to detect an envelope thereof; and an
envelope amplifier that amplifies an output of the envelope
detection circuit and supplies a bias voltage to the main
amplifier; an output of the quarter-wave length transmission line
and an output of the main amplifier being coupled together to
produce an output signal.
3. The power amplifier according to claim 1, wherein the main
amplifier is biased for class AB, and the peak amplifier is biased
for class C.
4. The power amplifier according to claim 2, wherein the main
amplifier is biased for class AB, and the peak amplifier is biased
for class C.
5. The power amplifier according to claim 1, wherein the peak
amplifier has a constant drain voltage and is turned on only at a
peak power level of the modulation wave input signal to amplify the
modulation wave input signal, and wherein the main amplifier is
operated with a low voltage at an average level of the modulation
wave input signal and is operated with a high voltage at an
instantaneous peak level of the modulation wave input signal.
6. The power amplifier according to claim 2, wherein the peak
amplifier has a constant drain voltage and is turned on only at a
peak power level of the modulation wave input signal to amplify the
modulation wave input signal, and wherein the main amplifier is
operated with a low voltage at an average level of the modulation
wave input signal and is operated with a high voltage at an
instantaneous peak level of the modulation wave input signal.
7. The power according to claim 1, wherein the main amplifier
allows for a load variation from a low impedance condition to a
high impedance condition, by the impedance transforming action of
the quarter-wave length transmission line, as the input level
increases; the low impedance condition being an impedance condition
which affords the maximum efficiency for the low voltage operation
at an average level of the modulation wave input signal, the high
impedance condition being an impedance condition which affords the
maximum output for the high voltage operation at a peak output.
8. The power according to claim 2, wherein the main amplifier
allows for a load variation from a low impedance condition to a
high impedance condition, by the impedance transforming action of
the quarter-wave length transmission line, as the input level
increases; the low impedance condition being an impedance condition
which affords the maximum efficiency for the low voltage operation
at an average level of the modulation wave input signal, the high
impedance condition being an impedance condition which affords the
maximum output for the high voltage operation at a peak output.
Description
REFERENCE TO RELATED APPLICATION
This application is based upon and claims the benefit of the
priority of Japanese patent application No. 2008-006086 filed on
Jan. 15, 2008, the disclosure of which is incorporated herein in
its entirety by reference thereto.
FIELD OF THE INVENTION
This invention relates to a power amplifier and, more particularly,
to an amplifier with which a high efficiency characteristic may be
achieved even under an operating state with a large back-off from a
saturation output.
DESCRIPTION OF RELATED ART
In a large capacity and high speed mobile communication system, a
modulation wave signal having a high peak factor (peak power to
average power ratio) is used, as in W-CDMA of the third-generation
mobile communication system. With the amplifier used in such
system, in which a high peak power is to be amplified, it is
necessary to provide a sufficient back-off from the saturation
output, thus presenting a problem that the amplifier efficiency is
lowered.
A large variety of amplifiers have so far been proposed for
improving the efficiency in an operating range with a large
back-off.
The drain bias of a drain-bias modulation amplifier, operating in
the vicinity of the saturation output, is modulated with being
tuned to a modulation wave input signal. This yields a peak power
as well as a high efficiency operation.
Referring to FIG. 4, a drain-bias modulation amplifier, shown in
Non-Patent Document 1, includes, in a drain-bias circuit 7 of a
main amplifier 3, an envelope amplifier 5 and an envelope detector
4. The envelope amplifier 5 modulates the drain bias (Vd) in
accordance with an envelope of the modulation wave input signal. By
adjusting a delay line 6, provided on the input side of the main
amplifier 3, the drain bias may be modulated with being tuned to
the modulation wave input signal.
With the above configuration, the drain-bias modulation amplifier
of FIG. 4 operates with high efficiency at low voltage for an
average power level of the modulation wave signal, while operating
at high voltage for a peak power level of the modulation wave
signal to amplify a peak signal.
As another example of the high efficiency amplifier, Non-Patent
Document 2 discloses a Doherty amplifier shown in FIG. 5. With the
Doherty amplifier, an output of a main amplifier (Main Amp.) 3
biased for class AB operation, and an output of a peak amplifier
(Peak Amp.) 8 biased for class C operation, are combined by a
quarter-wave length (.lamda./4) transmission line 15 (Doherty
network) with a wavelength one-fourth the fundamental frequency.
The quarter-wave length transmission line 15 is connected to an
output side of the main amplifier 3.
The Doherty amplifier operates by taking advantage of the impedance
transformation by the quarter-wave length transmission line 15.
That is, at a low input level, with the peak amplifier 8 in
OFF-state, only the main amplifier 3 operated at high efficiency in
a load state in which load impedance is twice the load impedance RL
(2*RL), with the back-off being low. At a high input level, the
peak amplifier 8 is turned on, while the load on the main amplifier
3 is decreased, that is, changed from 2*RL to RL. Moreover, the
peak amplifier 8 earns the peak power of the amplifier as a whole.
Thus, a high efficiency operation in the high back-off region may
be enabled as a high peak power is maintained.
FIG. 6 shows comparison of the output back-off to efficiency
characteristics of a drain-bias modulation amplifier, a Doherty
amplifier and a class-B amplifier. Theoretically, the drain-bias
modulation amplifier and the Doherty amplifier may be improved over
the class-B amplifier in the efficiency of the output region with
approximately 6 dB back-off from the saturation output.
Thus, to improve the power efficiency of an amplifier appreciably
without deteriorating the system signal quality, such an amplifier
exhibiting both a high efficiency characteristic and a high peak
power characteristic in combination in its operation level has been
desired.
With a view to achieving an operation of higher efficiency at an
operating level of a large back-off from the saturation output,
there is shown in Patent Document 1 (Example 2) an example of a
Doherty amplifier provided with an input level detector that
detects an input level. The power supply voltages of a main
amplifier and a peak amplifier are controlled in dependence upon an
output signal of an input level detector. The peak amplifier is
termed in Patent Document 1 as an auxiliary amplifier.
FIG. 7 shows a circuit configuration disclosed in Example 2 (FIG.
4) of Patent Document 1. In the configuration of FIG. 7, there is
shown a Doherty amplifier made up of a main amplifier 22 biased for
class AB operation, an auxiliary amplifier (peak amplifier) 23
biased for class B operation, and a quarter-wave length
transmission line (impedance transformation unit) 24 connected to
the main amplifier 22. The outputs of the two amplifiers are
combined together by the quarter-wave length transmission line. The
power supply voltages of the main amplifier 22 and the auxiliary
amplifier 23 are controlled by a voltage controller 26 in response
to an output signal of an input level detector 25. In case of a low
input level, the power supply voltage is lowered to maintain a high
operating efficiency and, in case of a high input level, the power
supply voltage is raised to increase the maximum output power of
the amplifier in its entirety, thereby assuring a high efficiency
at the operating point. [Non-Patent Document 1] Donald F. Kimball
et al., "High-Efficiency Envelope-Tracking W-CDMA Base-Station
Amplifier Using GaN HFETs", IEEE Trans. Microw. Theory Tech. vol.
54, No. 11, November 2006 [Non-Patent Document 2] I. Takenaka et
al., "A 240 W Doherty GaAs power FET amplifier with high efficiency
and low distortion for W-CDMA base stations", 2004 IEEE MTT-S Int.
Microwave Symp. Dig., pp. 525-528 [Patent Document 1] JP Patent
Kokai JP-A-2007-81899 (FIG. 4)
SUMMARY OF THE DISCLOSURE
The following analysis of the related art is given by the present
invention.
If, in the above-described Doherty amplifier, a drain bias
modulation circuit is provided on the main amplifier side, and the
power supply is modulated with being tuned to an input signal,
there is presented a problem that the efficiency at an average
operating level is lowered, and a saturation output on the main
amplifier is also lowered. In short, it has been found that a high
efficiency operation and a high peal power cannot be achieved in
combination.
FIG. 8 shows an example of drain voltage dependence of a load-pull
characteristic of a field effect transistor used in an amplifier
(Vds=12V, 48V). Referring to FIG. 8, the load condition for the
maximum efficiency for a low drain voltage (Vd1=12V) (Eff. Match)
differs from the load condition for the maximum output for a high
drain voltage (Vd2=48V) (Pout match) insofar as actual devices are
concerned.
Due to drain voltage dependence of reactance components proper to a
transistor, the optimum load impedance for a low drain voltage
becomes lower than that for a high drain voltage.
FIG. 9 shows the drain voltage dependence of the output
(Pout)-efficiency characteristic in case of setting an output side
load condition to an optimum load impedance for a high drain
voltage (Vds=8V to 48V). FIG. 10 shows drain voltage dependence of
the output-efficiency characteristic in case of setting the output
side load condition to an optimum load impedance for a low drain
voltage.
A voltage-dependent characteristic of the envelope of the
output-efficiency characteristic may be obtained by modulating the
drain voltage in proportion to the amplitude of the modulation wave
input signal.
FIG. 11 shows an example of the probability distribution of
amplitude components of a W-CDMA modulation wave signal.
The maximum instantaneous peak power has a wave crest value which
is high and is on the order of 10 dB from the average power having
the highest distribution probability.
If the load condition on the output side of the amplifier
transistor is set to the optimum load impedance for a high drain
voltage, the efficiency at an average output level (28 dBm) with a
back-off of approximately 10 dB from the peak output of 38 dBm is
low and is on the order of 60% against the maximum efficiency of
73% at a peak output.
If conversely the load condition on the output side of the
amplifier transistor is set to the optimum load impedance for the
low drain voltage, the efficiency at a high average output level of
73% may be obtained, as may be seen from FIG. 10. However, the peak
output is 36 dBm, which is lower by as much as 2 dB than if the
load condition is set to the optimum load impedance at a high drain
voltage.
That is, there is presented a problem that, if the output side load
condition is set to the maximum output condition for the high
voltage operation, the efficiency at the average operating level is
lowered, and that, if the output side load condition is set to the
maximum output condition for the low voltage operation, the peak
power characteristic is lowered.
It is now assumed that the drain-bias modulation amplifier,
performing the operation as described above, is applied to a
Doherty amplifier as shown in Example 2 of Patent Document 1 (see
FIG. 7 hereof). In such case, since the peak amplifier is turned on
with increase in the input level, the load impedance of the main
amplifier is changed from high impedance to low impedance due to
the impedance transformation operation of the quarter-wave length
transmission line.
Thus, even though the optimum load impedance during the low drain
voltage operation for the low input level is on the low impedance
side and the optimum load impedance during the high drain voltage
operation for the high input level is on the high impedance side,
the operation of the amplifier becomes the transformation of two
opposite load impedance states.
That is, in the Doherty amplifier, having such drain bias
modulation configuration, in which the tendency of the drain
voltage dependence of the optimum load impedance of the device is
not in keeping with the amplifier operation, even when the drain
voltage is increased with increase in the output signal level. It
is thus difficult to achieve in combination high efficiency
characteristic and the high peak power characteristic at the
operating point level.
In addition, in such Doherty amplifier, since the drain bias
modulation circuit is provided on the side of the peak amplifier as
well, marked gain reduction takes place due to a resonance point
generated in an operating range of the amplifier, with the result
that an optimum amplifier characteristic may not be obtained.
FIG. 12 shows a gain-frequency characteristic of a Doherty
amplifier in case of changing drain voltages of the main and peak
amplifiers. With decrease in the drain voltage, the resonant point
shifts into the operating range of from 1.9 to 2.05 GHz, thus
appreciably decreasing the gain.
The reason may be such that, since the output impedance of the
class C transistor of the peak amplifier exhibits marked drain
voltage dependence which is higher than that of the class AB
transistor of the main amplifier, pass loss increases under the
influence of the impedance on the side of the peak amplifier when
the drain voltage is decreased at an output combining point (point
A of FIG. 5) between the main amplifier and the peak amplifier.
Inherently, the impedance looking from the output combining point
of the main and peak amplifiers (point A of FIG. 5) towards the
peak amplifier is desirably an open state. However, when the drain
voltage is changed, the output impedance of the peak amplifier is
changed and deviated from the open state, when the drain voltage is
changed, with the result that output power leakage occurs from the
main amplifier, thereby causing loss.
That is, if, in the Doherty amplifier, the drain bias of the peak
amplifier is modulated, the gain characteristic is lowered under
the effect of bias dependence of the class C transistor in the peak
amplifier.
The present invention seeks to solve one or more of the above
problems.
In one aspect of the present invention, there is provided a power
amplifier comprising a main amplifier, a peak amplifier, and a
quarter-wave length (.lamda./4) transmission line connected to an
output of the peak amplifier. The quarter-wave length transmission
line has a length which is equal to one-fourth of the wave-length
(.lamda.) of a fundamental frequency. The main amplifier includes a
drain-bias circuit. The drain-bias circuit includes: an envelope
detector that receives a modulation wave input signal to detect an
envelope thereof; and an envelope amplifier that modulates a drain
bias of the main amplifier in accordance with the envelope of the
modulation wave input signal.
According to the present invention, there is also provided a power
amplifier comprising: a main amplifier; a peak amplifier, an input
distributor for distributing the power of a modulation wave input
signal to the main amplifier and to the peak amplifier; a quarter
wave line connected to an output of the peak amplifier, and a bias
circuit that affords a bias voltage to the main amplifier. The bias
circuit includes an envelope detection circuit that receives the
modulation wave input signal to detect an envelope thereof, and an
envelope amplifier that amplifies an output of the envelope
detection circuit to deliver a bias voltage to the main amplifier.
An output of the quarter wave line and an output of the main
amplifier are combined together and output.
According to the present invention, the peak amplifier operates at
a constant drain voltage and is turned on only at a peak power
level of the modulation wave signal to amplify the signal. The main
amplifier operates with a low voltage at an average level of the
modulation wave input signal, while operating with a high voltage
at an instantaneous peak level of the modulation wave input
signal.
According to the present invention, the main amplifier allows for a
load variation from a low impedance condition to a high impedance
condition with increase in the input level by the impedance
transforming action of the quarter-wave length transmission line.
The low impedance condition is such a condition that affords the
maximum efficiency for the low voltage operation at an average
level of the modulation wave input signal. The high impedance
condition is such a condition that affords the maximum output for
the high voltage operation at a peak output.
According to the present invention, the high efficiency
characteristic and the high peak power characteristic may be
obtained in combination at the operating level of the
amplifier.
Still other features and advantages of the present invention will
become readily apparent to those skilled in this art from the
following detailed description in conjunction with the accompanying
drawings wherein only exemplary embodiments of the invention are
shown and described, simply by way of illustration of the best mode
contemplated of carrying out this invention. As will be realized,
the invention is capable of other and different embodiments, and
its several details are capable of modifications in various obvious
respects, all without departing from the invention. Accordingly,
the drawing and description are to be regarded as illustrative in
nature, and not as restrictive.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram showing a configuration of a power
amplifier according to an Example of the present invention.
FIG. 2 is a diagram showing load variations of a main amplifier and
a peak amplifier in the power amplifier of the Example of the
present invention.
FIG. 3 is a graph showing an output-efficiency characteristic in
the power amplifier of the Example of the present invention.
FIG. 4 is a circuit diagram showing a configuration of a drain-bias
modulation amplifier of Non-Patent Document 1.
FIG. 5 is a circuit diagram showing a configuration of a
conventional Doherty amplifier of Non-Patent Document 2.
FIG. 6 is a graph for comparison of output back-off to efficiency
characteristics of a drain-bias modulation amplifier, a Doherty
amplifier and a class-B amplifier.
FIG. 7 is a circuit diagram showing a configuration of an amplifier
shown in Example 2 of Patent Document 1.
FIG. 8 is a diagram showing drain voltage dependence of a load-pull
characteristic of a field-effect transistor.
FIG. 9 is a graph showing drain voltage dependence of the
output-efficiency characteristic in case of fixation to optimum
load impedance for a high drain voltage.
FIG. 10 is a graph showing drain voltage dependence of the
output-efficiency characteristic in case of fixation to optimum
load impedance for a low drain voltage.
FIG. 11 is a graph showing probability distribution of an average
power to peak power ratio of a W-CDMA modulation wave signal.
FIG. 12 is a graph showing a gain-frequency characteristic in case
of changing drain voltages of the main amplifier and the peak
amplifier in a conventional Doherty amplifier.
PREFERRED MODES OF THE INVENTION
According to the present invention, an output of a main amplifier
(3) biased for class AB operation and an output of a peak amplifier
(8) biased for class C operation are combined by a quarter-wave
length (.lamda./4) transmission line (9) of a frequency equal to
one-fourth the fundamental frequency. This quarter-wave length
transmission line is connected to an output side of the peak
amplifier (8). The main amplifier (3) has a drain-bias circuit (7)
which is provided with an envelope amplifier (5) and an envelope
detection circuit (4). The envelope amplifier (5) modulates a drain
bias voltage in accordance with an envelope of the modulation wave
input signal.
The main amplifier (3) allows for a load variation from a low
impedance condition to a high impedance condition, by the impedance
transforming action of the quarter-wave length transmission line
(9), provided at an output side of the peak amplifier (8), with
increase in the input level. The low impedance condition is such a
condition that affords the maximum efficiency for the low voltage
operation at an average output level. The high impedance condition
is such a condition that affords the maximum output for the high
voltage operation at a peak output.
Since the bias of the peak amplifier (8) is fixed, the gain
characteristic is not affected by the impedance of the peak
amplifier (8). According to the present invention, a high
efficiency characteristic and a high peak power characteristic may
be achieved in combination at the operating point level. The reason
is that the tendency of the drain voltage dependence of the optimum
load impedance of the device is in keeping with the amplifier
operation. The present invention will now be described with
reference to Examples.
FIG. 1 depicts a circuit configuration of a power amplifier
according to the present invention. A main amplifier 3 biased for
class AB operation and a peak amplifier 8 biased for class C
operation are connected to a quarter-wave length (.lamda./4)
transmission line 9 provided on an output side of the peak
amplifier 8. The quarter-wave length transmission line 9 has a
length equal to one-fourth of the wave-length (.lamda.) of the
fundamental frequency.
A drain-bias circuit 7 for the main amplifier 3 includes an
envelope amplifier 5 and an envelope detector 4. The envelope
amplifier modulates the drain bias of the main amplifier 3 in
accordance with an envelope of a modulation wave input signal. The
envelope detector 4 includes a diode.
The modulation wave input signal is delayed by a delay line 6 and
supplied to an input distributor 11. The input distributor 11
distributes the power of the modulation wave input signal to the
main amplifier 3 and the peak amplifier.
The input distributor 11 may be a quarter-wave length transmission
line as that provided in the Doherty amplifier.
The peak amplifier 8 operates at a constant drain voltage and is
turned on only at a peak power level of the modulation wave signal
to amplify a signal. The detailed circuit configuration is shown in
Non-Patent Document 1.
By the drain-bias circuit 7, the main amplifier 3 is operated with
a low voltage at an average level of the modulation wave signal and
is operated with a high voltage at an instantaneous peak level of
the modulation wave signal, respectively.
The quarter-wave length transmission line 9 is provided on the side
of the peak amplifier 8 in a manner contrary to the case of the
Doherty amplifier explained above as a related art technique (see
FIG. 5, for instance). That is, by the impedance transforming
action of the quarter-wave length transmission line 9, provided on
the side of the peak amplifier, the main amplifier 3, allows for a
load variation from a low impedance condition to a high impedance
condition with increase in the input level. The low impedance
condition is a condition that affords the maximum efficiency for
the low voltage operation at an average level of the modulation
wave input signal, whilst the high impedance condition is such a
condition that affords the maximum output for the high voltage
operation at a peak output.
Since the bias of the peak amplifier 8 is fixed, the gain
characteristic is not affected by the impedance of the peak
amplifier 8.
Thus, even if, in the amplifier of the present invention, the drain
voltage is increased with increase of the input signal level, a
high efficiency characteristic and a high peak power characteristic
may be achieved in combination at an operating point level because
the tendency of the drain voltage dependence of the optimum load
impedance of the device may be brought into coincidence with the
amplifier operation.
In the present Example, an output of the main amplifier 3 biased
for class AB operation and an output of the peak amplifier biased
for class C operation are combined together by the quarter-wave
length transmission line 9 provided on the output side of the peak
amplifier 8. The quarter-wave length transmission line is of a
length equal to one-fourth of the wave-length of the fundamental
frequency. Further, the drain bias voltage of the main amplifier 3
is modulated in accordance with an envelope of the modulation wave
input signal. It is thus possible to realize optimum load
variations that follow an input signal level in keeping with drain
voltage dependence of the optimum load condition in the device.
In this manner, the operation efficiency of an amplifier may
drastically be improved as the high peak power characteristic is
maintained.
FIG. 2 shows the manner of a load variations of transistors of the
main and peak amplifiers brought about by increase in the input
signal level.
The load impedance of the main amplifier of the present Example is
changed, with the increase of the input level, from the low
impedance state to the high impedance state. In the low impedance
state, the maximum efficiency for the low voltage may be obtained,
whereas, in the high impedance state, the maximum output for the
high voltage may be obtained.
FIG. 3 shows an output-efficiency characteristic in the amplifier
of the present Example. Comparison of the characteristic of the
present Example to that of the amplifier of the related art
technique, shown in FIGS. 9 and 10, reveals that, with the
amplifier of the present Example, a peak output of 38 dBm may be
obtained, while the average output level (28 dBm) at a back-off of
approximately 10 dB is high and 73%, as shown in FIG. 3, thus
achieving a high efficiency characteristic.
This may be due to the drain-bias circuit 7 connected to the main
amplifier 3 and the impedance transforming action of the
quarter-wave length transmission line 9 connected to the output of
the peak amplifier 8. Specifically, the load impedance is changed
in such a manner that, at an average output level, when the peak
amplifier is OFF, the main amplifier 3 is operated in a low voltage
state under a low impedance condition in which the maximum
efficiency is obtained at a low voltage. At a peak output, when the
peak amplifier 8 is ON, the main amplifier 3 is operated in a high
voltage state under a high impedance condition in which the maximum
output is obtained at the high voltage.
Also, since the bias of the peak amplifier 8 is fixed, the gain
characteristic is not affected by the impedance of the peak
amplifier 8.
Thus, with the amplifier of the amplifier of the present invention,
even if the drain voltage is increased with increase in the input
signal level, the high efficiency characteristic and the high peak
characteristic may be achieved in combination at the operating
point level. The reason is that the tendency towards the drain
voltage dependence of the optimum load impedance of the device is
keeping with the amplifier operation.
The disclosures of the aforementioned Non-Patent Documents 1 and 2
and Patent Document 3 are incorporated by reference herein. The
particular examples may be modified or adjusted within the gamut of
the entire disclosure of the present invention, inclusive of
claims, based on the fundamental technical concept of the
invention. Further, variegated combinations or selections of the
elements disclosed herein may be made within the framework of the
claims. That is, the present invention may encompass various
modifications or corrections that may occur to those skilled in the
art within the gamut of the entire disclosure of the present
invention, inclusive of claim and the technical concept of the
present invention.
It should be noted that other objects, features and aspects of the
present invention will become apparent in the entire disclosure and
that modifications may be done without departing the gist and scope
of the present invention as disclosed herein and claimed as
appended herewith.
Also it should be noted that any combination of the disclosed
and/or claimed elements, matters and/or items may fall under the
modifications aforementioned.
* * * * *